Most people agree that hypnosis does something to your brain — specifically something that makes people make fools of themselves at hypnotist shows. But how does it actually affect the human brain? Can it make people recall events perfectly? Are post-hypnotic suggestions a bunch of baloney? What is the truth about hypnotism?

Nearly every culture in the world has a history of hypnotic trances. Some only considered them spiritual or eerie, but most began to make use of them as soon as they were discovered. India and China have ancient records showing hypnotic trances being used to relieve pain during surgery. The practice migrated to Europe, where in 1794 a young boy having an operation for a tumor was put under. The boy was Jacob Grimm, who grew up to write about quite a few hypnotic trances in his and his brother's book of fairy tales.

As ether and anesthesia came in, hypnosis went out. The medical community at large rejected its claims to pain reduction and hypnotic suggestions. Meanwhile, Hollywood embraced it as a plot device, adding on fantastic properties that made it seem still more outlandish to the public. It finally settled in the entertainment industry, where it does have the power to make people do extremely silly things, with extreme sincerity. (Watching some dead-serious kids give Grammy speeches as if they were Ricky Martin convinced me that hypnotism must have power over people.) But the extent of its power has always been debated.

How Hypnosis Affects the Brain

A person in a hypnotic state will appear tuned-out, and one of the marks of true hypnosis is a decrease in involuntary eye movement to the point where deeply hypnotized people will have to be reminded to blink. This gives an observer the impression that the hypnotized aren't paying attention. In fact, they're playing hyper-attention. Compared to a resting brain, many areas come online when a person is put into a hypnotic trance. All the areas that flare to life during hypnosis are also engaged when a person is concentrating on mental imagery — except one. Like many areas of the brain, the precuneus lights up during many different tasks, all of them having to do with a consciousness of self. It also deals with visuospatial aspects of the brain, letting us know where we are in space.

In essence, when we're hypnotized, people are able to concentrate intensely on self-created imagery (or imagery that suggested to them) but do not place their selves as part of that imagery. They've lost the reminder of what they personally do and what normal judgments they make, while increasing their ability to think about a whole range of imaginary situations. This explains the way adults can act out under the influence of hypnosis, or how they might remain calm and collected in situations that would otherwise terrify them. But how far does it go?

The Power of Hypnosis

One of the most incredible feats people under hypnosis are supposed to perform is the ability to remember details of a past event that a person has consciously forgotten. In movies everyone, under hypnosis, suddenly has a photographic memory (right up until they try to see the killer's face). There is debate, and some hypnotherapists claim that they have helped people retrace their steps through hypnosis and remember locations of, say, lost items or valuable papers.

But a larger study at Ohio State University cast doubt on whether hypnosis can actually enhance your memory to such an extent. When two groups of students, one hypnotized and one only relaxed, were asked about the dates of certain historical events, the groups performed equally well. The only difference was, when they were informed that there were some errors in their answers, the hypnotized group changed fewer answers than the unhypnotized group. Hypnosis got a more infamous reputation when it was used by psychologists to 'recover' lost memories, often of childhood abuse, that never happened.

But hypnosis does have the power to tap into memory in ways that other techniques do not. Most importantly, it has the ability to induce temporary, reversible amnesia. This condition is extremely rare, as many amnesiacs don't recover their memories, and some unlucky ones can't make new memories.

Although not all hypnotized patients can have their memories suppressed, and no one suppresses their memories unless they're told to, the effects can be startling. For one thing, the entire memory can be brought back with a word. This indicates that hypnosis doesn't obliterate memories, it just temporarily shuts off the retrieval system. One woman was told she couldn't remember the word 'six,' and so answered 'seven' to mathematical questions. A man forgot his own name. Any memory could be suppressed.

But the memory didn't go away. A group of students were hypnotized and told to forget a short film they had just watched. While unable to answer questions about the film, they had no problem remembering if the film was, for example, shot on a handheld camera. It was only the content that was suppressed. This ability to remember and react to the context of a thing without remembering the thing itself is the post-hypnotic suggestion. It's a suggested habit that makes sense in context (like reaching for a cell phone when hearing a ringtone) but not at that moment (if you deliberately left your cell phone at home). It just doesn't occur to the person to think of what they're reacting to before they react.

Another amazing hypnotic ability is, supposedly, suppression of pain. While it makes sense that people might feel less self-conscious, what with the part of their brain that feels self-consciousness offline, and that their perception might be altered by the part of the brain that governs perception, but pain is different. One of the primary functions of pain is to force someone out of the reverie they're in and make them pay attention to reality. Pain is the outside world breaking in.

But scientists studying perception think our experience is shaped far more by what we expect the stimulus to be than the stimulus itself. There are ten times as many nerve fibers carrying information down as carrying it up. Most people will have experienced feeling a shape in their pockets and being disoriented until they remember that it's a wadded up receipt, at which point the sensations seem familiar.

More to the point, most people will remember an itching or sting that, when they see a more serious injury than they expected, will blossom into pain. A hypnotized person undergoing surgery, for example, may be able to convince themselves that they're experiencing the discomfort of a bug bite instead of a scalpel. That, along with a state of enforced relaxation, can make all the difference.

But the shadiest aspect of hypnotism — what it can make an entranced person do — is still shrouded in mystery. Most hypnotists take pains to stress that no one is enslaved when they're in a hypnotized state, and that they can't be made to do something they don't want to do. Of course, that is the line they'd take. Scientists are, understandably, reluctant to give people the suggestion to murder someone under hypnosis, and test the results. Perhaps the best test of this isn't science, but history. Although there have always been legends of people under the direction of an evil puppet-master committing unspeakable acts against their will, there have been no actual cases. So don't worry about going to those hypnotist shows. Just . . . don't sit in the front.